On paper F# seems pretty great. Unfortunately, in practice it was the first and so far the only language I've ever used that I never found myself enjoying.

I wrote a simple, but relatively robust toy language interpreter in it, so the time I put in wasn't trivial and I kept thinking it would get better, but no such luck. This is just my anecdotal experience though, I know some companies have been wildly successful with F# as the technical foundation for their entire platform. Jet is a great example of one such company.